1. Field of the Invention
This invention relates to improvements in ring laser gyroscopes and means for detecting an output therefrom.
2. Brief Description of the Prior Art
Laser gyroscopes are unconventional gyroscopes, as they do not contain a spinning mass, unlike the "spinning mass" gyroscopes of the prior art. In operation, a closed laser ring type cavity is established which supports independent oppositely directed traveling waves, each of which can oscillate at different frequencies. Each counter rotating beam frequency is dependent upon rotation of the cavity with respect to inertial space. The frequency difference between the counter rotating beams is a direct measure of the cavity rotation rate. Ordinarily, the output frequency difference is detected by superimposing one of the counter rotating beams onto the other to establish an interference pattern which moves at a rate equal to the frequency difference.
One optics-detector arrangement of the prior art is taught by D. M. Thymian and T. J. Podgorski, ASTIA Document AD 527867, GG1300AD01, Laser Gyro Final Report, May, 1973, pp. 7-8. A similar typical ring laser gyroscope embodiment is represented in the drawing of FIG. 1. The gyroscope 10 is fabricated about a closed triangular ring defined by reflective mirror elements 11, 12, and 13. Counter rotating light beams, denoted by the reference numerals 15C and 15CC travel around the ring in clockwise and counter clockwise directions, respectively. Typically, a lasing medium (not shown) is included within the cavity to support the counter rotating beams 15C and 15CC.
The output from the ring laser gyroscope is derived at two of its corners, part of the output being derived at the reflective surface 12, the other at reflective surface 13. With respect first to the output at reflective surface 13, the surface 13 is designed to be partially reflective and partially transmissive to the light impinging on it. Thus, the light 15C traveling in a clockwise direction, in addition to being reflected onto the reflective surface 11, is partially transmitted through the reflective surface 13 to follow a path generally indicated by the reference numeral 17 to impinge upon a detector 20. In a similar fashion, the light beam 15CC traveling in a counter clockwise direction is partially reflected from the surface 13 to impinge upon the reflective surface 12, and is partially transmitted through the reflective surface 13 to follow a path 18 to impinge upon a detector 21. The detectors 20 and 21 are intensity detectors, each of which produces an output which may be summed with the other to produce an overall loop or cavity intensity indication. (In some cases, it may also be advantageous to have the single beam intensity signals available also.) Because the intensity of the beams is dependent upon the length of the ring, typically at least one of the reflective surfaces 11, 12, or 13 is constructed to be adjustably movable in response to an intensity signal, such as is developed by the detectors 20 and 21 (moving means not shown). Thus, the length of the ring is dynamically tuned in accordance with the intensity of the light indicated by the output of the detectors 20 and 21.
In addition to the foregoing, ring laser gyroscopes of the prior art usually include a means for measuring the frequency difference between the clockwise and counter clockwise rotating beams. In the embodiment shown in FIG. 1, this means is provided at the corner reflector 12, and is constructed at follows. The corner reflector 12 has a partially reflective and partially transmissive coating 12A deposited on one of its faces. Thus, a portion of each of the clockwise and counter clockwise light beams incident upon the surface 12A is reflected onto the mirrors 13 and 11, respectively. In addition, a portion of each of the incident counter clockwise and clockwise light beams is transmitted through the reflective surface 12A to impinge respectively on a corner reflector 24 and a detector 25, as presently described. (It should be noted that if desired the locations of the corner reflector 24 and the detector 25 may be reversed from the positions presently to be described without adversly affecting the performance of the circuit.) More specifically, the transmitted portion of the clockwise rotating beam 15C follows a path 30 through the substrate 12B to impinge onto the detector element 25. The transmitted portion of the counter clockwise rotating beam 15CC follows a path 31 through the substrate 12B to be reflected by the corner reflector 24 back onto the back side of the substrate 12B. The back side of the substrate 12B is also coated with a 50-50 beamsplitter coating 26 (the beam splitting coating 26 covering approximately half of the substrate surface, as indicated) to reflect the light incident upon the surface 26 from the corner reflector 24 to impinge on the detector 25. The pattern of the light reflected from the beam splitting coating 26 is superimposed upon the pattern produced by the clockwise rotating beam 15C on the face of the detector 25. The superposition of the two beams produces an interference pattern by which the frequency difference of the two beams can be determined. One example of a typical gyro readout can be found in "The Laser Gyro", by F. Aronowitz, Laser Applications, Vol. 1, Academic Press, 1971, pp. 139-141.
In the construction of the corner reflector 12, typically a block or substrate 12B of optic material is provided onto which the partially reflective face 12A and beamsplitter face 26 are coated. In addition, as shown in the side view of the reflector element 12, denoted in FIG. 1 as 12S, the substrate 12 is shaped with a wedge configuration, as shown in cross section, whereby the partially reflective faces 12A and 26 are not parallel, but are displaced from a parallel position by a very small angle, .alpha.,. The angle, .alpha., which is on the order of, for example, thirty arc-seconds, determines the angle, .beta., between the respective beams 15C and 15CC onto the detector 25. By controlling the magnitude of the angle, .alpha., the separation of the interference fringes can be controlled, as is known in the art.
As shown in FIG. 2, the detector 25 includes a plurality of strips of detector material 33 placed on a substrate 34, and oriented generally in the direction of the stripes 36 produced by the interference pattern between the clockwise and counter clockwise rotating beams 15C and 15CC. The rate of movement of the stripes 36 across the detectors 33 is indicative of the frequency difference between the clockwise and counter clockwise rotating beams 15C and 15CC about an axis of rotational sensitivity, denoted by the outwardly pointing arrow 40 in FIG. 1. With the detector spacing approximately equal to one-fourth of the fringe spacing, it is possible to determine whether the fringes are moving up or down by techniques known in the art. The direction of fringe motion depends, of course, upon whether rotational input is clockwise or counterclockwise.
With respect to the prior art embodiment of the ring laser gyroscope of FIG. 1, it should be noted that two separate outputs are derived, an intensity output derived from the partially reflective element 13 at one corner, and a frequency difference indicating output derived at the partially reflective element 12 at another corner.
With respect to general design principles in ring laser gyroscopes, one important objective is to minimize the losses within the laser cavity or counter rotating beam path. Thus, ideally, partially reflective mirror surfaces should be avoided, if possible, because of their inherent losses. In addition, the substrates on which partially reflective coatings are deposited may be backed by special structures, such as piezoelectric assemblies, which makes the sampling of the output beams inconvenient, or, in some cases, impossible.